Phloem Loading of Sucrose: pH Dependence and Selectivity

Area of Science:

• Plant Physiology
• Molecular Biology
• Biochemistry

Background:

• Phloem loading is crucial for long-distance transport of sugars in plants.
• Understanding the mechanisms of sucrose uptake into the phloem is vital for plant productivity.

Purpose of the Study:

• To investigate the mechanism of sucrose uptake into the phloem of sugar beet (Beta vulgaris).
• To determine the influence of apoplastic pH on sucrose phloem loading.
• To elucidate the kinetics and pathway of sucrose accumulation in the phloem.

Main Methods:

• Autoradiography to track (14)C-sucrose distribution.
• Plasmolysis to assess cell volume changes and membrane integrity.
• (14)C-metabolite distribution studies to identify accumulated compounds.
• Kinetic analysis (K(m), V(max)) to characterize the loading process.
• Uptake studies with asymmetrically labeled sucrose to determine hydrolysis.

Main Results:

• The majority of exogenous (14)C-sucrose enters the phloem directly from the apoplast.
• Phloem loading is significantly inhibited at an alkaline apoplast pH (8) compared to acidic pH (5).
• The apparent K(m) of sucrose loading increases at alkaline pH, while V(max) remains unaffected.
• Sucrose is accumulated intact, not hydrolyzed by invertase before uptake.
• pH dependence observed in leaf discs correlates with phloem loading and translocation in intact leaves.

Conclusions:

• Sucrose phloem loading in sugar beet is a pH-dependent process.
• The findings support a model where sucrose loading is coupled to proton co-transport, aligning with the chemiosmotic hypothesis.
• This mechanism ensures efficient sucrose translocation from source leaves to other plant parts.